8 is standard but up to 10 for a new noid. 12 is still ok.
I run them up to 15 if I think it's faily so see if they are inconsist and then lowe them until the ball speed drops. 15, good go to 12, good go to 10 see where they start to stutter.
Remember the pin is supposed to be open 3.3 thou of a second so the time the solenoid is charged isn't as important as how long the air flows.